A known rotary cutting unit is disclosed in U.S. Pat. No. 6,244,148, and includes a rotary cutter in working relationship with a rotary anvil. The rotary cutter is provided with a substantially circular-cylindrical body having a surface and at least one knife member protruding from the surface, the radially peripheral part of the knife member having a diameter larger than that of the surface. Each side of the rotary cutter is provided with an axle supported in bearings. Between the axles and the surface, i.e. on each side of the surface, a pair of annular abutment members are provided. The abutment members have a diameter larger than that of the surface, in order to allow abutment against a pair of load receiving portions of the anvil.
The anvil is provided with an anvil portion and the pair of load receiving portions. The anvil portion is adapted to co-operate with the knife member of the rotary cutter, whereas the load receiving portions are adapted to abut the abutment members of the rotary cutter. The anvil is supported in bearings outside the anvil portion and outside the load bearing portions, seen in the axial extension of the anvil.
Furthermore, the abutment members have a diameter which is substantially the same as the radially peripheral part of the knife member. The abutment members are adapted to lie against and transmit loads such that a predetermined pressure is exerted on the load receiving portions of the anvil to achieve a desired cutting property. Optionally, the abutment may also transmit rotation of the rotary cutter drum to the anvil surface, such that it turns in a direction opposite to that of the rotary cutter. A product is cut from a web introduced between the drums by the centrally arranged knife member.
The described rotary cutter however suffers from the drawback that the portions of the knife members in the axial center of the rotary cutting drum do not cut as precisely as the portions of the knife members closer to the axial periphery thereof. This is due to the fact that the rotary cutting drum exerts a pressure onto the anvil surface via the abutment members, thereby causing the anvil to be bent. This is shown in FIG. 5, illustrating the principle of a prior art anvil A corresponding to the anvil disclosed in U.S. Pat. No. 6,244,148. A pair of load receiving portions B are arranged on either sides of the axial extension of an anvil portion C, whereas a pair of annular support surfaces D for co-operating with bearings are arranged on either sides of the pair of load receiving portions B. When load E is applied to the load receiving portions, the central portion of the anvil A will be bent slightly downwards as shown (exaggerated) by the broken line F due to the counter directed force G on the surface D, i.e. at the bearings. Such bending may be denoted negative bending.
Another prior art rotary cutting unit has an anvil for cooperation with a rotary cutting drum having two or more knife members arranged side by side. Such a rotary cutter not only suffers from the drawback described above, but also in that it has a long axial extension, causing the anvil drum and the rotary cutting drum to also be bent by gravity, i.e. the longer and heavier the anvil, the more it will be bent negatively by gravity that will add to the described effect.
The use of two or three parallel knife members or one large knife member on a long rotary cutter to co-operate with such an anvil will thus only have acceptable cutting properties at the peripheral portion of the anvil, whereas the knife member closer to the central portions may not cut through the web to be cut.